When a photocatalytically active material (to be sometimes simply referred to as “photocatalyst” hereinafter) is irradiated with energy of its band gap or higher, it is excited to generate electrons in a conduction band, and holes are generated in a valence band. And, it is known that the generated electrons reduce surface oxygen to generate super oxide anions (.O2−), that the holes oxidize surface hydroxyl groups to generate hydroxy radicals (.OH), and that these reactive activated oxygen species exhibit a strong oxidative decomposition function and highly efficiently decompose organic substances adhering to the photocatalyst surface.
Studies are being made of the application of the above functions of the photocatalyst, for example, to deodorization, antifouling, antibacterial protection and sterilization and further to decomposition and removal of those various substances in waste water or waste gas, which are problems to cause environmental pollution.
As another function of the photocatalyst, further, it is also known that a photocatalyst surface exhibits super-hydrophilic nature in which the contact angle thereof to water is 10° or less when the photocatalyst is optically excited, as is disclosed, for example, in International Patent Publication No. 96/29375. Studies are being made of the application of the above super-hydrophilic function of the photocatalyst to the prevention of the fouling caused, for example, on sound insulation walls along an expressway, illumination lamps in a tunnel, street lights, etc., by soot, etc., contained in emission gases from automobiles or the use of the photocatalyst for a film for a body coating or side-view mirror of an automobile or for a defogging or self-cleaning window mirror.
As the above photocatalyst, various compounds having semiconductor properties are known, and among these are metal oxides such as titanium dioxide, iron oxide, tungsten oxide and zinc oxide and metal sulfides such as cadmium sulfide and zinc sulfide. Of these, titanium oxide, particularly anatase type titanium dioxide, is useful as a practical photocatalyst. This titanium dioxide exhibits excellent photocatalytic activities by absorption of light having a specific wavelength in the ultraviolet region included in ordinary light such as sunlight.
When a photocatalyst layer is formed on an organic substrate such as a plastic substrate, there is caused a problem that the organic substrate inevitably deteriorates in a short period of time when the photocatalyst is directly coated thereon. In a photocatalyst film having a photocatalyst layer, for example, on a plastic film, therefore, an intermediate layer is generally formed for preventing the deterioration caused on the substrate film by photocatalytic activity and for improving adhesion to the substrate film. As the above intermediate film, there is generally employed an approximately several μm thick film made of a silicone resin, an acryl-modified silicone resin, or the like.
However, the above photocatalyst film is caused to have problems that it deteriorates in approximately 1 to 3 years and that the transparency thereof decreases due to an interference of the films or that the antifouling property thereof decreases. The above deterioration is presumably caused as follows. Since the above intermediate layer has an organic substituent, this organic component is decomposed by the photocatalytic activity, and as a result, the intermediate layer is caused to undergo cracking, or “floating” or partial separation occurs in an interface between the photocatalyst layer and the intermediate layer or between the intermediate layer and the substrate film to cause the interference. In the above photocatalyst film, further, there is another problem that since the intermediate layer is as thick as several μm, it is liable to cause partial interfacial separation or chipping due to the bending or flexing of the film per se, so that the interference is liable to occur.
On the other hand, the present inventors have found an organic-inorganic composite graded material having a composition that continuously changes in the thickness direction, which material is useful as a novel functional material in the various fields, for example, of a coating film, an adhesive for bonding an organic material and an inorganic or metallic material, an intermediate film that is formed between an organic substrate and a photocatalyst film and that prevents the deterioration of the organic substrate and an intermediate film that improves the adhesion of an organic substrate and an inorganic or metallic material (Japanese Patent Application No. 11-264592).
The above organic-inorganic composite graded material is an organic-inorganic composite material containing a chemical-bonding product of an organic polymer compound and a metallic compound and has a components-gradient structure in which the content of the above metallic compound continuously changes in the material thickness direction, and it is a novel material useful in the above various fields.
The present inventors have found that the deterioration of the organic substrate by the photocatalyst can be prevented by interposing, as a protective layer, a film of the above organic-inorganic composite graded material between the organic substrate and the photocatalyst layer when the photocatalyst layer is formed on the organic substrate, and a patent application directed to this art has been filed.
Since, however, the above composite graded film is formed of a transparent material, the effect of blocking sunlight including ultraviolet light cannot be expected, and the weather resistance of the substrate itself accounts for the lifetime of a product thereof. When the above composite graded film is formed, further, it is essential to apply a coating liquid containing an organic solvent, and the resistance of the substrate to the solvent is also an essential factor for the product lifetime.